The present invention relates to an image recording apparatus in which recording is conducted by a plurality of paralleled print heads in each of which a plurality of light emitting elements are linearly aligned in the primary scanning direction.
Conventionally, as an apparatus in which an image is recorded with plural-time exposure for each pixel by the use of a print head equipped with an array-shaped light source in which light emitting elements are linearly aligned in a direction (the primary scanning direction) perpendicular to the conveying direction (the secondary scanning direction) of the photosensitive material used as an image recording medium, an apparatus provided with a LED array, a vacuum fluorescent indicating tube or a liquid crystal shutter array is known.
Further, as an apparatus in which recording is conducted by a non-exposure system such as a melting heat transfer system or a sublimating heat transfer system, a color image recording apparatus provided with a thermal print head in which heat generating elements are aligned in an array form is known.
Also, a color image recording apparatus using three print heads provided with three kinds of light emitting elements different in the wavelength of emitted light such as red, green and blue is known. In the color image recording apparatus, each of the three print head records an image one by one on the same line of pixels in order to obtain a color image. Further, in order to increase the recording speed, a method of using a plurality of the same kind of print head is known.
Incidentally, in the image recording apparatus provided with a plurality of print heads, it may be difficult to perfectly conform the actual image resolution of each print head to others. Even if the same kind of print heads are provided with the same image resolution, the distances among pixels in the primary scanning direction may become different among the print heads due to manufacturing errors of the print heads.
As a result, since the positions of the recorded image (pixel) in the primary scanning direction become different among the print heads, for example, color blur may take place on the edge portion of an image in the case that the image is recorded with multi-colors. As the worst case, color deviation in which the positions of images are deviated for each color may occur, resulting in that image quality may be degraded greatly.
Now, the color blur and the color deviation are explained more with reference to FIG. 11. In FIG. 11, the positions of print heads are adjusted such that color blur and color deviation do not take place at the position of address 0, thereafter exposure recording is conducted in the primary scanning direction with a given pixel interval by the print heads. At the position of address 767 in the vicinity of the center of the scanning line, color blur and color deviation take place, and at the position of address 2559 corresponding to the right edge, the positions of the recorded pixels are completely deviated from each other.
In a print head such as a vacuum fluorescent indicating tube in which a mask pattern of elements of a line is produced with the use of photo resist, errors in image resolution are small. In contrast, with a long print head in which a plurality of blocks each composed of 128 pieces of light emitting elements such as LED are combined, the above image degradation noticeably take place.
Further, in the case that an exposure section is constructed by a combination of different types of print heads such as a LED print head and a vacuum fluorescent print head, even if the different types of print heads are indicated with the same size such as 300 dpi (dot/inch), there may be errors in image resolution smaller than 1.0%. Color blur and color deviation may be caused by the errors smaller than 1.0% to an easily noticeable extent depending on the size of image.
Further, even in a monochromatic image, in order to reduce density irregularities by averaging dispersion in luminance among light emitting elements, or in order to make the life of a print head longer by reducing the light emission amount per a single print head, plural pieces of print heads are used so as to form an image with superimposed pixels. In this case, color blur and color deviation may be caused by deviation in the recorded positions of the superimposed pixels.
The deviation in the recording positions in the primary scanning direction is stated above. Also, the deviation in the recording positions in the secondary scanning direction is a problem as same as in the primary scanning direction. As a popular case as the deviation in the recording positions in the secondary scanning direction, the deviation may be caused in the case that the recording timing in the secondary scanning direction is not appropriately set for the relationship between the conveying speed of an image recording medium and the distance between print heads. In addition, in the case that the degree of parallelism between print heads is not satisfied with a predetermined level, or in the case that the print head is warped or curved, the deviations in the recording position in the secondary scanning direction are varied depending on the position in the primary scanning direction. Also, when the position of the first recording element on each print head is not parallel to the secondary scanning direction, color deviation, namely color deviation in the primary scanning direction, may occur.
In addition, depending upon the accuracy of the conveyance speed of the image recording medium, or the accuracy of the conveyance position in the primary scanning direction, an image may not be recorded at a desired position on the recording medium. In the case of prints with margins, prints having different margin amounts between laterally and longitudinally may occur. In addition, when a borderless print (which is a common print format in a silver halide print), it was problematic that unnecessary margins occur. Although it may be possible to increase the degree of precision in manufacturing and assembling so as to minimize the deviation in the recording positions in the primary scanning direction and in the secondary scanning direction as small as possible, increasing the degree of precision of machinery parts and lowering in assembling efficiency may raise the manufacturing cost. Further, even if the deviation in the recording positions is adjusted as small as possible at the time of shipment, it may be difficult to avoid deviations in recording position taking place due to the following causes: working environment such as temperature and humidity, deterioration due to transportation or long term usage, fluctuation due to wear or friction force down in driving system in the secondary direction, physical property change due to material change of a recording medium, position change due to assembling or disassembling at the time of print head replacement or at the time of removing the jammed recording medium.